Gas blast circuit breaker



May 12, 1964 H. N. SCHNEIDER GAS BLAST CIRCUIT BREAKER 2 Sheets-5heet l Filed Aug. 8, 1960 Inventor: Harold N. Schneider,

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MaYlZ, 1964 H. N. SCHNEIDER 3,133,176

l GAS BLAST CIRCUIT BREAKER Filed Aug. 8, 1960 2 Sheets-Sheet 2 Inventor: Hanf-old N. Schneider,

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United States Patent O 3,133,176 GAS BLAST ClRCUIT BREAKER Harold N. Schneider, Springfield, Pa., assigner to General Eiectric Company, a corporation of New York Filed Aug. 8, 1960, Ser. No. 48,039 8 Claims. (Ci. 2050-148) This invention relates to a gas-blast circuit breaker of the type that is provided with voltage-controlling impedance means shunting its main contacts and, more particularly, relates to an improved switching arrangement for interrupting the current flowing through said impedance means after said main contacts have been opened.

For controlling the voltage developed across the main contacts of a circuit breaker during a circuit interrupting operation, it has been customary to shunt these contacts with an impedance through which current is transferred when the main contacts are opened to interrupt the circuit. For interrupting the current through the impedance, there is customarily provided a switch which opens after the main contacts have parted.

Such a switch must not only be capable of successfully interrupting the relatively heavy currents that flow through the impedance during the interruption of short circuit currents but must also be capable of interrupting without objectionable chopping the relatively light currents that llow through the impedance during the interruption of relatively light currents, such as magnetizing currents. By chopping is meant the abrupt extinction of the arc and cut-off of current prior to a natural current zero. If the amount of current chopped is excessively high, then possibly-damaging overvoltages can be induced across inductive devices connected in circuit with the interrupter.

It has been found that most switches that are capable of handling relatively high currents tend to chop excessively, whereas switches that are capable of interrupting light currents without excessive chopping tend to be inefiicient at the higher current levels.

Accordingly, an object of my invention is to provide a gas-blast circuit breaker of this general type with an auxiliary switch that is not only capable of interrupting the heavy currents flowing through the impedance during short-circuit interruption but is also capable of interrupting without excessive chopping the light currents flowing through the impedance during the interruption of magnetizing currents and the like.

In carrying out my invention in one form, `l provide a gas-blast circuit breaker that has an impedance shunting its main contacts. ln series with the impedance, separable auxiliary contacts are provided for establishing an auxiliary arc. Electrically connected to one of the auxiliary contacts is a conductive exhaust member of tubular form defining an exhaust passage leading from the region in which the auxiliary arc is established. A conductive probe is located in the exhaust passage, Suitable means is provided for producing a blast of gas through said exhaust passage that transfers the downstream terminal of the auxiliary arc to the probe and locates the are in a portion of the exhaust passage upstream from the probe where the blast extends in an axial direction with respect to the arc. The portion of the exhaust passage thaty is located upstream from the probe has a predetermined minimum cross-sectional area defining an upstream orifice through which the arc extends. The portion of the exhaust passage that is located downstream from the probe has a predetermined minimum cross-sectional area deining a downstream orifice. The upstream orifice has an effective area substantially greater than the effective area of the downstream orifice so that in the region upstream from the probe where the arc is located, the velocity of the flow is below sonic velocity. By limiting the velocity of the flow to a subsonic level, l am able to considerably reduce the amount of current chopped by the interrupter in comparison to current chopped at sonic velocities and yet am able to retain the ability of the interrupter to successfully handle relatively large currents.

For a better understanding of my invention, reference may be had to the following specification taken in connection with the accompanying drawings, wherein:

FIG. l is a side elevational view, partially in section, showing a circuit breaker embodying my invention. The circuit breaker is depicted in the closed position.

FIG. 2 is an enlarged sectional view of a portion of the circuit breaker of PEG. l showing the depicted part in a partially open position.

lG. 3 is a view partially in section taken along the line 3 3 of FIG. l.

FIG. 4 is a View taken along the line 4 4 of FIG. 2.

Referring now to FIG. 1, the circuit breaker shown therein is of the general type disclosed and claimed in Patent No. 2,783,338-Beatty and 2,9ll,546-Oppel, assigned to the assignee of the present invention. This circuit breaker comprises an enclosed interrupting chamber lll dened, in part, by a metallic casing l2 which is filled with a pressurized arc-extinguishing gas, such as air.

A pair of elongated conductive studs l5 and Ztl project into the casing i2 from diametrically-opposed points, and each of these studs carries a suitable stationary contact assembly lo at its radially inner end. Cooperating with each stationary contact assembly is a movable Contact 2li pivotally mounted upona stationary pivot 29. These pivots 29 are supported upon stationary brackets 3l which are integral with one end of a stationary operating cylinder 32. Suitable means (not shown) are provided for transferring current between the movable contacts Z8 and the brackets 3l, so that the brackets 31 together with the cylinder 32 form a conductive path electrically interconnecting the two movable contacts 2S.

The cylinder 32, at its left hand end, is suitably supported from a generally cylindrical housing 33, which, in turn, is suitably securedat its left hand end to the metallic casing 12. The mechanical connection between the housing 33 and the casing l2 is best shown in FIG. 3, where the housing 33 is shown provided with a flange $4 that is suitably joined to a flange 35 secured to the casing 12.

For producing a gas-blast action for extinguishing the arcs which are established by separation of the contacts ldand 28, the housing 33 is provided with a normallyclosed annular exhaust passage 36 which leads from the interrupting chamber 1l to the surrounding atmosphere. This will be more readily apparent from FIG. 3. The housing 33 at its right hand end is formed with a pair of generally diametrically-opposed nozzle-type electrodes 3S defining inlets to the exhaust passage 36. For controlling the flow of arc-extinguishing gas through the nozzle electrodes 38 and through the exhaust passage 3d, there is provided at the outer end of the exhaust passage 36 a cylindrically-shaped reciprocable blast valve member 4l) which slides smoothly in a surrounding tubular valve housing fil integrally formed in the housing 33. In FIG. 3, the valve member 40 is shown in its closed position wherein an annular flange 42 formed at its left hand end sealingly abuts against the stationary flange 34, which serves as a valve seat. The valve member lill is normally maintained in this closed position of FIG. 3 by the action of a suitable spring (not shown) and by the action of the pressurized gas within the passageway 36. This ygas produces upon the flange 42. an unbalanced force urging the valve' member 4t) to the left into its closed position.

Since the chamber 11 is normally filled with pressurized gas, it will-be apparent that when the valve member di) is opened by movement to the right (by means not shown), gas in the chamber 11 will ilow at high speed through the nozzles 38 and out the passage 3o past valve member 40 to atmosphere, as is indicated by the arrows e and f shown in FIG. 3. This rapid ilow of gas through the nozzles 3S creates an axial arc-enveloping blast which acts rapidly to extinguish the arcs which are drawn adjacent the nozzles by movement of the movable contacts 28 away from their stationary contact assemblies llo.

For operating the blast valve di) and the movable contacts 28, a combined operating mechanism preferably of the huid-actuated type shown in the aforementioned Beatty patent is provided within the cylinders 32 and 33. The details of this operating mechanism form no part of the present invention and, hence, such details are not shown in the present application. An adequate understanding of the present invention may be had if it is understood that the operating mechanism acts during an opening operation to drive a piston rod shown at 5S to the right and also acts to open the blast valve l-. The piston rod 5S is coupled to the contacts 2S (by means soon to be described) and, hence, such movement of the piston-rod serves to drive the contacts open. At a predeterminer instant after the contacts 23 have been opened, the thenopen blast valve 40 is driven closed by the operating mechanism, as is described in detail in the Beatty patent. This prevents further loss of gas from the rinterrupting chamber 11.

The means for coupling the piston rod 53 to the main contacts comprises a crosshead 59 and two sets of connecting links 60. The crosshead 59 is rigidly secured to the piston rod 5S by suitable clamping means, whereas the connecting links 6) are pivotally connected at 61 and 62 to the crosshead and movable contacts, respectively.

In the position of FIG. l, the movable contacts 23 are biased into closed position by means of overcenter compression springs 64. Each oi these springs 64 has one end pivotally supported at e5 on a projecting portion of one of the brackets 31. At their inner ends, the springs 64 are pivotally supported on the crosshead 59. These overcenter springs 64 tend to urge the contacts closed while the crosshead 59 is to the left of a reference line connecting the pivots 65. But when the crosshead is moved to the right beyond this reference line (as occurs during a contact-opening operation), the overcenter springs thereupon tend to urge the contacts in a contactopening direction. This action coupled with that of the operating mechanism acts to hold the contacts 28 in a fully-open position until the operating mechanism is subsequently operated to close the contacts 28.

Shunting the upper pair of main contacts 16, 23 is an impedance element 70, preferably in the form of a resistor wound about an insulating core 70a carried by a conductive tube 71. The conductive tube 71 is supported `from the conductive stud 20 by means of conductive webs 72 electrically interconnecting the tube 71 and the stationary contact assembly 16. The lower terminal of the.

resistor '70 is connected to the tube 71, whereas the upper terminal is locally insulated from the tube 71 and is connected by means of a conductor 74 to an electrode '73 of an impedance or resistor switch 75.

The resistor switch '75 comprises, in additon to the electrode '73, a second electrode 76 spaced from electrode 73 and coacting with electrode 73 to form an interrupting gap between the two electrodes. The electrode '73 is supported on the central housing 33 by an insulator '77 which is capable of electrically isolating the electrode 73' from the housing 33 when the resistor switch is open. The electrode 73 is also supported on the housing 33 but is electrically connected to the housing. Electrically bridging the two stationary electrodes 73 and 76 is a movable electrode 78, which in its closed position of PEG. l, butts against the two stationary electrodes. Thus, it will be seen that the resistor '7u is connected in shunt with the upper Contacts 16, 23 by means of a circuit which extends through the parts 72, '71, '7th-74, 73, 7%, 76 and 33. As will be apparent from FIG. l, the lower main contacts 16, 2S are shunted by a similar circuit. Since the parts forming this lower shunting circuit are substantially identical to those forming the upper circuit, corresponding lower parts have been assigned corresponding reference numerals followed by the sufx a.

The electrodes of the resistor switch may be thought of as being auxiliary contacts for the overall circuit breaker and are frequently referred 'to in the present application as auxiliary contacts.

The electrode '76 is an annular member which is located at the upstream end of tubular structure defining an auxiliary exhaust passageway 1G@ leading into the main exhaust passage 36. No how takes place through this auxiliary exhaust passage ltlll so long as the blast valve 40 at the downstream end of the main exhaust passage is closed. But when the blast valve 4t) opens during a circuit interrupting operation, pressurized gas flows from the interior of casing 11, first through the auxiliary exhaust passage 105i and then through the main exhaust passage 3e and the blast valve all@ to the low pressure regionoutside the casing 12.

The mechanism for operating the resistor switch 75 may be of any conventional form but is preferably of the type shown and claimed in the aforementioned Oppel patent. The details of this mechanism form no part of the present invention and hence will not be explained in the present application. It is sufcient for the purposes of the present application merely to understand that the movable contacts '78 and 73a are driven to the right to separate them from their respective stationary electrodes 73, 7S and 73a, 7&1 at a suitable instant after the main contacts 16 have opened. This separation of the two sets of auxiliary contacts occurs substantially simultaneously inasmuch as the movable auxiliary contacts '78 and 78a are coupled together through a cross bar ll of insulating material which is mechanically coupled to the main ccntacts of the interrupter through means including a rod S5. The coupling between the movable auxiliary contact 78 and the cross bar S1 comprises a pin 84 extending freely through an opening in the cross bar 81 and an abutment 83 secured to the outer end of the pin 84. A compression spring S2 is disposed between the cross-bar and the movable auxiliary contact 78. When the crossbar is driven to the right, it impacts against the abutment d3 and drives the pin 8l) together with its connected movable contact 7h to the right out of engagement with the stationary electrodes 73 and 7 Separation of the upper contacts of the resistor switch establishes an arc between the stationary electrode 73 and the movable contact or electrode 78. Just prior to the time that the arc is drawn between the electrodes 73 and 78 of the auxiliary switch, the blast valve 40 had opened and thus a blast of gas had begun flowing through the auxiliary exhaust passage 1th), the main exhaust passage 36 and the blast valve 49. This blast of gas quickly drives the lower terminal of the arc established between the electrodes 73 and 73 off of the electrode 78 and onto a probe Si) of generally circular cross-section 1ocated in the exhaust passage 14H9. The approximate position of the arc when the lower terminal has been transferred to the probe Sti is depicted at 161 in the enlarged view of FIG.V2, where the gas blast (depicted by the arrows B) is shown streaming past the arc in an axial direction relative to the arc. The arc then extends across the arcing gap between contact 73 and the exhaust member 76 and through the portion of the exhaust passage located upstream from the probe 80.

The probe Sti and the portion of the exhaust passage 1% upstream from the probe titl are so proportioned that the velocity of the gas flowing upstream from the probe is below sonic velocity. Thus, when the arc is in the position of FIG. 2, it is positioned substantially entirely in a region of subsonic flow. The importance of this relationship will soon be described in greater detail, but, first, a description will be given of the proportions that are responsible for limiting the velocity of the iiow to a subsonic level. Referring to FIG. 2, the portion of the exhaust passage 100 located upstream from the probe {if} has a region A2 of minimum cross-sectional area, and the portion of the exhaust passage y100 located downstream from the upstream face of the probe S has a region A1 of minimum cross-sectional area. The region A2 may be thought of as an upstream orifice and the re gion A1 may be thought of as a downstream orifice. The effective cross-sectional area of A2 is equal to the actual area of A2 times the usual discharge coeicient for an orifice of this configuration. The actual cross-sectional area of A1 is equal to the cross-sectional area of the exhaust passage surrounding the probe 80 minus the crosssectional area of the probe itself. The effective crosssectional area of A1 is equal to this actual area times the usual discharge coefficient for an orifice of this configuration. To limit the gas velocity upstream from the probe 80 to a subsonic level, the effective area of upstream orifice A2 must be greater than .53 times the effective area of downstream orifice A1. In the disclosed interrupter, the effective area of upstream orifice A2 is several times larger than that of downstream orifice A1, and this results in velocities considerably below sonic velocity upstream from the probe 80. It is to he understood, however, that subsonic velocities may be obtained with smaller upstream orifices, providing the effective area of upstream orifice A2 is still greater than .53 times the effective area of the downstream orifice A1. If the effective area of A2 is about equal to .53 times the effective area of A1, sonic velocity will occur upstream from the probe S0. As is well known, decreases in the effective area of the upstream orifice below .53 times the effective area of the downstream orifice will result in no substantial further increases in velocity above the sonic level.

I have found that by limiting the gas velocity through the arcing region to a subsonic level, I am able to effect considerably reductions in the amount of current that will be chopped during low current interruptions.

While it is true that a gas type interrupter has little tendency to chop in static air, static air is subject to the disadvantage that it has a relatively low current interrupting capacity in comparison to that of a stream or blast of air. Under short circuit conditions, the auxiliary interrupter of the present application may be called upon y to interrupt currents of 750 or more amperes, and such currents cannot be readily interrupted in static air. However, I am able to interrupt such currents with comparative ease by using a subsonic blast of air, which is capable not only of interrupting these relatively high resistor currents but is also capable of interrupting relatively low magnetizing currents without excessive chopping.

In certain cases it may be desirable to provide a series of holes extending axially through the probe 80 in order to reduce any accumulation of arcing products ahead of the probe. AIn such cases, the actual area of the downstream orifice would be the sum of the areas of these holes and the area of A1 as depicted in FIG. 2.

It is to be understood that substantially the same arcextinguishing action takes place at the lower auxiliary contacts as at the upper auxiliary contacts. Hence, no detailed description is given of the arc-extinguishing action at the lower auxiliary contacts.

While I have shown and described a particular ernbodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing `from my invention in its broader aspects, and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

vl. In a gas blast circuit breaker, separable main contacts, impedance means shunting said main contacts, separable auxiliary contacts connected in series with said impedance means for establishing an auxiliary arc in series with said impedance means, a conductive exhaust member of generally tubular form electrically connected to one of said auxiliary contacts and spaced from the other of said auxiliary contacts by an arcing gap, said tubuiar exhaust member defining an exhaust passage leading yfrom the region in which said auxiliary arc is established, a conductive probe located in said exhaust passage, means for producing a blast of gas through said exhaust passage that transfers the downstream terminal of said arc to said probe and to a position wherein said arc extends across said arcing gap and through the portion of sai-d exhaust passage upstream from said probe, said blast extending in an axial direction with respect to said arc when said arc terminal is on said probe, the portion of said exhaust passage located upstream from said probe having a predetermined minimum cross-sectional area deiining an upstream orifice through which said arc extends when its downstream terminal is on said probe, the portion of said exhaust passage located downstream from said probe having a predetermined minimum cross-sectional area defining a downstream orifice, velocity-control means for limiting the velocity of the liow of gas along substantially the entire length of the arc to a subsonic level that produces an effective reduction in the currentchopping 'level as compared to that present with sonic velocity, said velocity-control means comprising said upstream and downstream orifices and being characterized by the effective cross-sectional area of said upstream orifice being at least as large as the effective cross-sectional area of said downstream orifice, the effective cross-sectional area of said upstream orifice being sufficiently low that the velocity through said upstream orifice is high enough toeffect interruption of at least several hundred amperes of current through said impedance means.

2. The circuit breaker of claim l in which the effective area of said upstream orifice is substantially larger than one times the effective area of said downstream orifice.

3. The circuit breaker of claim l inwhich said downstream orifice is an annular orifice having its inner circumference defined by the outer periphery of said probe and its outer circumference defined by the portion of said exhaust passage surrounding said probe.

4. In a gas blast circuit breaker, an enclosure containing pressurized gas, separable main contacts located in said pressurized gas, impedance means shunting said main contacts, separable auxiliary contacts connected in series with said impedance means for establishing within said pressurized gas an auxiliary arc in series with said impedance means, a conductive exhaust member of generally tubular form electrically connected to one of said auxiliary contacts and spaced `from the other of said auxiliary contacts by an arcing gap, said tubular exhaust member defining an exhaust passage leading from the region in which said auxiliary arc is established, a conductive probe located in said exhaust passage, means including a blast valve located downstream from said probe for producing a blast of gas through said exhaust passage that transfers the downstream terminal of said arc to said probe and to a position wherein said arc extends through the portion of said exhaust passage upstream from said probe and across said arcing gap, said blast extending in an axial direction with respect to said arc when said downstream arc terminal is on said probe, the portion of said exhaust passage located upstream from said probe having a predetermined minimum cross-sectional area defining an upstream orifice through which said arc extends when its downstream terminal is on Said probe, the portion of said exhaust passage located downstream from said probe having a predetermined minimum cross-sectional area defining a downstream orifice, velocity-control means for limiting the velocity of the flow through said upstream orifice to a level that is substantially below sonic velocity and that produces an effective reduction in the current chopping level as compared to that present with sonic velocity, said velocity-control means comprising said upstream and downstream orifices and being characterized by the effec-` tive Cross-sectional area of said upstream orifice being at least as large as the effective cross-sectional area of said downstream orifice, the effective cross-sectional area of said upstream orifice being sufficiently low that the velocity through said upstream orifice is -high enough to effect interruption of at least several hundred amperes of current through said impedance means.

5. IIn a gas blast circuit breaker', separable main contacts, impedance means shunting said main contacts, means comprising a pair of auxiliary electrodes connected in series with said impedance means for establlishing an auxiliary arc in series with said impedance means, an exhaust member of generally tubular form defining an exhaust passage extending lengthwise of said auxiliary arc, one end of said tubular exhaust member being spaced from one of said auxiliary electrodes by means or" a gap, said auxiliary arc extending upstream from the other of said auxiliary electrodes through said exhaust passage and across said gap, means for producing a blast of gas through said exhaust passage that extends in an axial `direction with respect to said arc when the downstream terminal of said arc is on saidother auxiliary electrode, the portion of said flow passage located upstream from said other auxiliary electrode having a predetermined minimum cross sectional area defining an upstream orifice through which said arc extends when its downstream terminal is on said other auxiliary electrode, the portion of said flow passage located downstream from said other auxiliary electrode having a predetermined minimum cross sectional area defining a downstream orifice, velocity-control means for limiting the velocity of the flow of gas along substantially the entire length of the arc to a subsonic level that produces an effective reduction in the current-chopping level as compared to that present with sonic velocity, said velocity-control means comprising said upstream and downstream orifices and being characterized bythe effective cross-sectional area of said upstream orifice being at least as large as the effective cross-sectional area of said downstream orifice, the ef'- fective cross-sectional area of said upstream orifice being sufiicientlylow that the velocity through said upstream orifice is high enough to effect interruption of at `least several'hundred amperes of current through said impedance means.

6.' The circuit breaker Aof claim 5 in which the effective area of said upstream orifice is substantially larger than onetimes the effective areal of said downstream orifice.

7. The circuitbreaker ofV claim 5 in which the effective area of said upstream orifice is several times larger than the effective area of said downstream orifice.

8. in a gas blast circuit breaker, separable main contacts, impedance means shunting said main contacts, means comprising a pair of auxiliary electrodes connected in series with said impedance means for establishing an auxiliary arc in series with said impedance means, an exhaust iember of generally tubular 'form defining an exhaust passage extending lengthwise of said auxiliary arc, one end of said tubular exhaust member being spaced from one of said auxiliary electrodes by means of a gap, said auxiliary are extending upstream from the other of said auxiliary electrodes through said exhaust passage and across said gap, means for producing a blast ofgas through said exhaust passage that extends in an axial direction with respect to said arc when the downstream terminal of said arc is on said other auxiliary electrode, the portion of said exhaust passage located upstream from said auxiliary electrode having apredetermined minimum cross-sectional area defining an upstream orifice through which said arc extends when its downstream terminal vis on said other auxiliary electrode, the portion of said exhaust passage located downstream from said auxiliary electrode having a predetermined minimum cross-sectional area defining a downstream orifice, and velocity-control means for limiting the velocity of the flow of gas along substantially the entue length of said arc to a level substantially below sonic velocity that produces an effective reduction in the current-chopping level as compared to that present with sonic velocity, said velocity-control means comprising said upstream and downstream orifices and being characterized by the effective cross-sectional area of -said upstream orifree being at least as ylargeas the effective cross-sectional area of said downstream orifice, the effective cross-sectionalA area of said upstream orifice being sufficiently low that the velocity through said upstream orifice is high enough to effect interruption of at least several hundred amperes of current through said impedance means.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,934 Flurscheim Jan. 23, 1945 2,574,334 Latour Nov. 6, 1951 2,897,324 Schneider July 28, 1959 2,911,546 Oppel Nov. 3, 1959 

1. IN A GAS BLAST CIRCUIT BREAKER, SEPARABLE MAIN CONTACTS, IMPEDANCE MEANS SHUNTING SAID MAIN CONTACTS, SEPARABLE AUXILIARY CONTACTS CONNECTED IN SERIES WITH SAID IMPEDANCE MEANS FOR ESTABLISHING AN AUXILIARY ARC IN SERIES WITH SAID IMPEDANCE MEANS, A CONDUCTIVE EXHAUST MEMBER OF GENERALLY TUBULAR FORM ELECTRICALLY CONNECTED TO ONE OF SAID AUXILIARY CONTACTS AND SPACED FROM THE OTHER OF SAID AUXILIARY CONTACTS BY AN ARCING GAP, SAID TUBULAR EXHAUST MEMBER DEFINING AN EXHAUST PASSAGE LEADING FROM THE REGION IN WHICH SAID AUXILIARY ARC IS ESTABLISHED, A CONDUCTIVE PROBE LOCATED IN SAID EXHAUST PASSAGE, MEANS FOR PRODUCING A BLAST OF GAS THROUGH SAID EXHAUST PASSAGE THAT TRANSFERS THE DOWNSTREAM TERMINAL OF SAID ARC TO SAID PROBE AND TO A POSITION WHEREIN SAID ARC EXTENDS ACROSS SAID ARCING GAP AND THROUGH THE PORTION OF SAID EXHAUST PASSAGE UPSTREAM FROM SAID PROBE, SAID BLAST EXTENDING IN AN AXIAL DIRECTION WITH RESPECT TO SAID ARC WHEN SAID ARC TERMINAL IS ON SAID PROBE, THE PORTION OF SAID EXHAUST PASSAGE LOCATED UPSTREAM FROM SAID PROBE HAVING A PREDETERMINED MINIMUM CROSS-SECTIONAL AREA DEFINING AN UPSTREAM ORIFICE THROUGH WHICH SAID ARC EXTENDS WHEN ITS DOWNSTREAM TERMINAL IS ON SAID PROBE, THE PORTION OF SAID EXHAUST PASSAGE LOCATED DOWNSTREAM FROM SAID PROBE HAVING A PREDETERMINED MINIMUM CROSS-SECTIONAL AREA DEFINING A DOWNSTREAM ORIFICE, VELOCITY-CONTROL MEANS FOR LIMITING THE VELOCITY OF THE FLOW GAS ALONG SUBSTANTIALLY THE ENTIRE LENGTH OF THE ARC TO A SUBSONIC LEVEL THAT PRODUCES AN EFFECTIVE REDUCTION IN THE CURRENTCHOPPING LEVEL AS COMPARED TO THAT PRESENT WITH SONIC VELOCITY, SAID VELOCITY-CONTROL MEANS COMPRISING SAID UPSTREAM AND DOWNSTREAM ORIFICES AND BEING CHARACTERIZED BY THE EFFECTIVE CROSS-SECTIONAL AREA OF SAID UPSTREAM ORIFICE BEING AT LEAST AS LARGE AS THE EFFECTIVE CROSS-SECTIONAL AREA OF SAID DOWNSTREAM ORIFICE, THE EFFECTIVE CROSS-SECTIONAL AREA OF SAID UPSTREAM ORIFICE BEING SUFFICIENTLY LOW THAT THE VELOCITY THROUGH SAID UPSTREAM ORIFICE IS HIGH ENOUGH TO EFFECT INTERRUPTION OF AT LEAST SEVERAL HUNDRED AMPERES OF CURRENT THROUGH SAID IMPEDANCE MEANS. 